1. Field of the Invention
The present invention relates to a radiation image detection apparatus for detecting a radiation image by a radiation image detector including multitudes of pixels disposed two-dimensionally, each having a TFT switch, and a residual charge amount estimation method for use with the apparatus. The invention also relates to a computer program product for causing a computer to perform the residual charge amount estimation method.
2. Description of the Related Art
Radiation image detection system having a TFT (Thin Film Transistor) active matrix array on which a radiation sensitive layer is disposed, and is capable of tentatively storing latent image charges generated according to radiation transmitted through a subject and reading out the stored latent image charges as electrical signals are known.
First, the structure of a conventional radiation image detection system will be described with reference to FIG. 11. FIG. 11 is a schematic equivalent circuit of 4×4 pixels. In FIG. 11, the reference numerals 111, 112, 113, and 114 respectively denote a capacitor, a TFT switch, a scanning line, and a data line.
Radiation, such as X-rays or the like, entered from a direction normal to the surface of FIG. 11 are converted to charges in the radiation sensitive layer and stored in each of the capacitor 111. Thereafter, the stored charges are transferred to the data lines 114 connected to either one of the source/drain electrodes of the TFT switches 112 by sequentially activating the TFT switches 112 by the scanning lines 113, and detected by the signal detectors 115.
In such type of radiation image detection system, the stored charges are erased by the readout operation and the like, but if the erasure of the stored charges is incomplete, remaining charges are retained within the radiation image detector as residual charges, which are gradually decreasing over a comparatively long time. If a next imaging is performed while the residual charges are not completely erased, the retained residual charges are detected together with the image signals of next imaging, which becomes an obstacle for an accurate measurement of inputted amount of radiation.
In order to solve this problem, for example, Japanese Unexamined Patent Publication No. 2003-010159 proposes a method for estimating the amount of residual charges remaining in a radiation image detector using pixel values of a radiation image obtained by the radiation image detector, and correcting pixel values of a radiation image obtained by next imaging.
If a large amount of radiation exceeding a predetermined value is irradiated on a certain region of the pixel array of the radiation image detector, an excessive amount of latent image charges generated according to the radiation is stored in each of the pixels within the region. This causes an increased amount of residual charges retained in the radiation image detector, while the pixel values are limited to the upper limit saturation value of the signal range and unable to obtain dose information greater than the saturation value. Thus, the method for estimating the amount of residual charges using pixel values described in Japanese Unexamined Patent Publication No. 2003-010159 is not able to accurately estimate the amount of residual charges when the amount of inputted radiation exceeds the predetermined value.
In view of the circumstances described above, it is an object of the present invention to provide a radiation image detection apparatus capable of estimating the amount of residual charges remaining in the radiation image detector even when a large amount of radiation exceeding a predetermined value (a value by which the pixel value is saturated) is irradiated on the radiation image detector, and a residual charge amount estimation method for use with the apparatus. It is a further object of the present invention to provide a computer program product for causing a computer to perform the residual charge amount estimation method.